An effective, low bit rate speech coder should have the characteristics of high speech intelligibility, speaker independence, ease of real time implementation and short throughput delay. To maintain low bit rate transmission and simultaneously achieve these goals is conventionally considered contradictory.
Various speech encoding algorithms and techniques have been proposed for encoding and decoding low data rate speech parameters from and to speech signals. Techniques for vector quantization of line spectrum pairs (LSP) data converted from standard linear predictive coding (LPC) parameters derived from input speech signals has been suggested, for example, in "Application of Line-Spectrum Pairs to Low Bit Rate Speech Encoders" by G. S. Kang and L. J. Fransen, Naval Research Laboratory, at Proceedings ICASSP, 1985, Pages 244-247. A tree encoding technique using adaptive or time varying quantization was disclosed by N. S. Jayant and S. A. Christensen, Bell Laboratories at IEEE Transactions on Communications, COM-26, September 1978, Pages 1376-1379. For transmitted speech signals encoded by vector quantization an improvement in decoding performance at the receiver end by optimization of the codebook for decoding words from the incoming signals has been disclosed in the prior art. See an article entitled "Improving the Codebook Design for Vector Quantization" by Y. J. Liu, ITT Defense Communication Division at Proceedings IEEE Military Communications, 1987, Pages 556-559. See also U.S. Pat. No. 4,975,956 and U.S. Pat. No. 5,012,518 both entitled LOW-BIT RATE SPEECH CODER USING LPC DATA REDUCTION PROCESSING issued on Dec. 4, 1990 and Apr. 30, 1991, respectively to Y. J. Liu et al. and assigned to the assignee herein. For more detail in regard to speech recognition systems, reference is also made to the following materials which are incorporated herein: "Keyword Recognition Using Template Concatenation", by A. L. Higgins and R. E. Wohlford, 1985 ICASSP; "Speaker Recognition by Template Matching", by A. L. Higgins, Proceedings of Speech Technology 1986, New York, N.Y.; "Improved Speech Recognition in Noise", by B. P. Landell, R. E. Wohlford, and L. G. Bahler, 1986 ICASSP, vol. 1, no. 1; U.S. Pat. No. 4,720,863 issued Jan. 19, 1988 to K. P. Li and E. H. Wrench; and copending U.S. patent application No. 346,054, filed on May 2, 1989, by B. P. Landell et al., entitled "Automatic Speech Recognition System Using Seed Templates", now U.S. Pat. No. 4,994,983.
As one can ascertain, many of the prior art proposals do not provide high intelligibility and reliability at low data rates. This is particularly true for speech independent speech coding in communications over high frequency channels in difficult environments.
Thus, it is an object to provide an improved speech compression system which circumvents the problems in the prior art.
It is a further object to provide a speech compression system which operates at 50 bits per second (BPS) and hence is capable of extremely low frequency operation with improved reliability and intelligibility.